For packaging food, medical and pharmaceutical products and miscellaneous goods, packaging bags using various plastic films are frequently used; packaging bags prepared by laminating heat-sealable non-oriented plastic in two or three layers on biaxially stretched plastic films are widely used.
Among plastic films, biaxially stretched polyester films are excellent in durability, moisture proofness, mechanical strength, heat resistance and oil resistance, are produced by the methods such as a tubular method, a flat-type simultaneous biaxial stretching method and a flat-type successive biaxial stretching method, and are widely used in the fields such as the field of food packaging.
However, packaging bags using biaxially stretched polyester films are unfortunately poor in tear openability. There is a method for imparting a notch in order to improve the openability; however, tearing from a notch frequently results in a phenomenon such that no linear tearing occurs, and consequently, the contents in the packaging bags are scattered to be wastes; additionally, soft confectionaries such as cookies are cracked at the time of opening packages, and when the contents are liquids, troubles such as staining of clothes may occur at the time of opening packages.
As easily openable materials excellent in the tear linearity such that when a film is torn, the film is torn linearly, there are materials prepared by laminating with a uniaxially stretched polyolefin film serving as an intermediate layer. Examples of such materials include a three-layer laminate film of biaxially stretched polyester film/uniaxially stretched polyolefin film/nonstretched polyolefin film; however, this three layer laminate film requires to be purposely provided with an intermediate layer, thus has a problem involving cost, and has been limited in the use thereof.
Accordingly, the present applicant has previously discovered, as a method for imparting the tear linearity to a biaxially stretched polyester film itself, a method in which a polybutylene terephthalate (modified PBT) including a polytetramethylene glycol (PTMG) having a molecular weight of 600 to 4000 in a proportion of 5 to 20% is mixed with polyethylene terephthalate (PET), in a ratio of modified PBT/PET=30/70 to 5/95 (Patent Literature 1 to Patent Literature 6).
Additionally, the present applicant has proposed a biaxially stretched polyester film and a laminate having tear linearity while having a high level of barrier property against oxygen or water vapor, on the basis of the vapor deposition of a metal compound such as aluminum oxide or silicon oxide at least on one side of a biaxially stretched polyester film having tear linearity (Patent Literature 7 and Patent Literature 8).
However, the biaxially stretched polyester film imparted with tear linearity is larger in the amounts of the outgases remaining in the film as compared to usual polyester films. Accordingly, this film is not able to be used as the film constituting the containers or packages for housing electronic materials such as semiconductors, required to have high cleanliness. Additionally, this film unfortunately involves a drawback such that the vapor deposition processing of this film in a vacuum atmosphere tends to cause the instability of the quality of the vapor deposited layer with the increase of the roll length.